Disable Battery charging! Why?

On Dell's users guides you read: HINT: To disable battery charging, you can use the Battery Charge Disable feature in Dell ControlPoint. This is useful, for example on airplanes, where charging of batteries may not be allowed.

I don't get it. I have never seen a wall outlet on airplanes to be able to connect to power line. And even if it existed, why you would take out your charger, connect it to the outlet and your laptop, then go to dell battery metter and then disable the charging?! So this feature can't be used on airplanes. What else? Why do you really want to connect to a power line and then disable the charging?